1. Field of the Invention
The present invention relates to bulk-mode resonator structures and manufacturing methods.
2. Discussion of the Related Art
To form time bases, many circuits use quartz oscillators. Such oscillators have a high quality factor on the order of 100,000 and a temperature-stable resonance frequency. They however have the disadvantage of a resonance frequency range limited to values below some hundred megahertz, typically 30 MHz. Further, they are difficult to integrate in a same assembly as electronic circuits formed in a semiconductor substrate.
To reach higher frequencies and decrease power consumption levels, theoreticians have provided to replace quartz oscillators with micro-electromechanical systems (MEMS), for example, bulk mode resonators.
FIG. 1A is a partial simplified top view of a bulk mode resonator. FIGS. 1B and 1C are cross-section views along planes B-B and C-C.
The resonator comprises a resonant element 1 generally formed of a portion of a single-crystal or poly-crystal semiconductor material, for example, having the shape of a bar of rectangular cross-section. Element 1 is attached to at least one anchor area 2 by arms 4. Arms 4 are arranged to contact element 1 at the level of its vibration nodes. Arms 4 are aligned along a neutral vibration line 5 of element 1 illustrated in dotted lines.
Apart from its connection with arms 4, element 1 is surrounded with an empty space 8. Two electrodes 10 and 11 are placed symmetrically in front of element 1 on either side of neutral line 5.
Cross-section plane B-B is parallel to neutral line 5 and does not cross arms 4. Cross-section plane C-C is perpendicular to plane B-B and cuts element 1 and electrodes 10 and 11.
As illustrated in FIGS. 1B and 1C, the resonator is formed in a thin single-crystal silicon layer resting on a silicon wafer 13 with an interposed insulating layer 15. The portion of space 8 separating element 1 from support 13 results from the partial removal of insulator 15. Element 1, anchors 2, and electrodes 10 and 11 are formed in the thin layer.
The resonator operates as follows. Element 1 is at least partially made of a conductive material set to a first voltage and electrodes 10 and 11 are set to a second voltage. The voltage difference between element 1 and electrodes 10 and 11 creates electrostatic forces which cause a deformation of element 1. Element 1 then enters a mode of bulk vibration at its resonance frequency, which corresponds to a bulk wave oscillation around central neutral line 5 of element 1. The deformation of element 1 causes a variation of the capacitance of the capacitor formed by element 1 and electrodes 10 and 11. This capacitance variation may be detected at the level of electrode 10 or 11.
Theoretically, it is thus possible to obtain resonators having resonance frequencies which vary within a range from between 10 and 300 MHz up to between 1.5 and 3 GHz.
Such resonators have the theoretical advantages of having lower power consumption levels than quartz oscillators and of being easily integrable.
In practice, the use of such bulk mode resonators, especially as time bases, comes against various limitations. In particular, uncontrollable variations of the effective resonance frequency with respect to the desired nominal frequency can be observed. The observed variations typically range within a range from 5 to 10% of the value of the desired frequency, randomly below or above this frequency.
Resonators having high frequencies greater than some hundred megahertz are particularly desired for time bases placed in portable devices such as telephones or computers. In such devices, a drift of the nominal frequency is unacceptable.
Various solutions have been provided to overcome the drift of the effective frequency of a bulk mode resonator with respect to a desired frequency.
A solution comprises performing an electric correction of the frequency, especially by modifying the bias voltage of element 1. This solution only enables correcting the frequency value by a few ppm/V. This is insufficient to correct the observed 5 to 10% drift or requires unrealistic bias levels for battery-powered devices.
Another solution comprises using the frequency-vs.-temperature variation property. Thus, a filament is deposited on the resonant element to modify—that is, decrease—the frequency. Such a solution is not satisfactory since the heating of the filament increases the device power consumption, which is not desirable in the case of battery-powered devices.
Other solutions comprise modifying the mass of element 1.
To decrease this mass, part of the body of element 1 may be vaporized by means of a laser. This correction is relatively complex to implement since it imposes using dedicated equipment which does not belong to devices currently used in the manufacturing of MEMS. The increased complexity and cost of this solution make it inapplicable in an industrial environment.
To increase the mass, it has been provided to submit element 1 to an ion bombarding. However, this solution lacks flexibility since it needs to be implemented before the device packaging.
All these solutions come up against the problem of predicting the occurrence and the extent of the drift, which can at least for the most part be imputed to the fact that the nominal conditions are not achieved and that the conditions of implementation of the previously-described manufacturing method are not easily repeated. In particular, in the definition of resonant element 1 by etching of the thin layer on insulator, from one batch to another, fluctuations of the composition of the etch medium or of its exposure time may occur, which results in a modification of the dimensions of element 1. Such fluctuations and others, which would seem to be negligible, combine to modify the frequency, which essentially depends on the dimensions, on the density, and on the Young's modulus of element 1.